The present invention generally relates to oxygen electrodes utilized in a variety of fuel cells. More particularly, the present invention relates to oxygen electrodes chemically impregnated with varying catalysts.
As the world""s population expands and its economy increases, the atmospheric concentrations of carbon dioxide are warming the earth causing climate change. However, the global energy system is moving steadily away from the carbon-rich fuels whose combustion produces the harmful gas. Experts say atmospheric levels of carbon dioxide may be double that of the pre-industrial era by the end of the next century, but they also say the levels would be much higher except for a trend toward lower-carbon fuels that has been going on for more than 100 years. Furthermore, fossil fuels cause pollution and are a causative factor in the strategic military struggles between nations. Furthermore, fluctuating energy costs are a source of economic instability worldwide.
In the United States, it is estimated, that the trend toward lower-carbon fuels combined with greater energy efficiency has, since 1950, reduced by about half the amount of carbon spewed out for each unit of economic production. Thus, the decarbonization of the energy system is the single most important fact to emerge from the last 20 years of analysis of the system. It had been predicted that this evolution will produce a carbon-free energy system by the end of the 21st century. The present invention is another product which is essential to shortening that period to a matter of years. In the near term, hydrogen will be used in fuel cells for cars, trucks and industrial plants, just as it already provides power for orbiting spacecraft. But, with the problems of storage and infrastructure solved (see U.S. application Ser. No. 09/444,810, entitled xe2x80x9cA Hydrogen-based Ecosystemxe2x80x9d filed on Nov. 22, 1999 for Ovshinsky, et al., which is herein incorporated by reference and U.S. patent application Ser. No. 09/435,497, entitled xe2x80x9cHigh Storage Capacity Alloys Enabling a Hydrogen-based Ecosystemxe2x80x9d, filed on Nov. 6, 1999 for Ovshinsky et al., which is herein incorporated by reference), hydrogen will also provide a general carbon-free fuel to cover all fuel needs.
A dramatic shift has now occurred, in which the problems of global warming and climate change are now acknowledged and efforts are being made to solve them. Therefore, it is very encouraging that some of the world""s biggest petroleum companies now state that they want to help solve these problems. A number of American utilities vow to find ways to reduce the harm done to the atmosphere by their power plants. DuPont, the world""s biggest chemicals firm, even declared that it would voluntarily reduce its emissions of greenhouse gases to 35% of their level in 1990 within a decade. The automotive industry, which is a substantial contributor to emissions of greenhouse gases and other pollutants (despite its vehicular specific reductions in emissions), has now realized that change is necessary as evidenced by their electric and hybrid vehicles.
Hydrogen is the xe2x80x9cultimate fuel.xe2x80x9d In fact, it is considered to be xe2x80x9cTHExe2x80x9d fuel for the future. Hydrogen is the most plentiful element in the universe (over 95%). Hydrogen can provide an inexhaustible, clean source of energy for our planet which can be produced by various processes. Utilizing the inventions of subject assignee, the hydrogen can be stored and transported in solid state form in trucks, trains, boats, barges, etc. (see the ""810 and ""497 applications).
A fuel cell is an energy-conversion device that directly converts the energy of a supplied gas into an electric energy. Researchers have been actively studying fuel cells to utilize the fuel cell""s potential high energy-generation efficiency. The base unit of the fuel cell is a cell having an oxygen electrode, a hydrogen electrode, and an appropriate electrolyte. Fuel cells have many potential applications such as supplying power for transportation vehicles, replacing steam turbines and power supply applications of all sorts. Despite their seeming simplicity, many problems have prevented the widespread usage of fuel cells.
Presently most of the fuel cell R and D focus is on P.E.M. (Proton Exchange Membrane) fuel cells. The P.E.M. fuel cell suffers from relatively low conversion efficiency and has many other disadvantages. For instance, the electrolyte for the system is acidic. Thus, noble metal catalysts are the only useful active materials for the electrodes of the system. Unfortunately, not only are the noble metals costly, they are also susceptible to poisoning by many gases, and specifically carbon monoxide (CO). Also, because of the acidic nature of the P.E.M fuel cell, the remainder of the materials of construction of the fuel cell need to be compatible with such an environment, which again adds to the cost thereof. The proton exchange membrane itself is quite expensive, and because of its low conductivity, inherently limits the power performance and operational temperature range of the P.E.M. fuel cell (the PEM is nearly non-functional at low temperatures, unlike the fuel cell of the instant invention). Also, the membrane is sensitive to high temperatures, and begins to soften at 120xc2x0 C. The membrane""s conductivity depends on water and dries out at higher temperatures, thus causing cell failure. Therefore, there are many disadvantages to the P.E.M. fuel cell which make it somewhat undesirable for commercial/consumer use.
The conventional alkaline fuel cell has some advantages over P.E.M. fuel cells in that they have higher operating efficiencies, they use less expensive materials of construction, and they have no need for expensive membranes. The alkaline fuel cell also has relatively higher ionic conductivity in the electrolyte, therefore it has a much higher power capability. Unfortunately, conventional alkaline fuel cells still suffer from certain disadvantages. For instance, conventional alkaline fuel cells still use expensive noble metals catalysts in both electrodes, which, as in the P.E.M. fuel cell, are susceptible to gaseous contaminant poisoning. While the conventional alkaline fuel cell is less sensitive to temperature than the PEM fuel cell, the active materials of conventional alkaline fuel cell electrodes become very inefficient at low temperatures.
Fuel cells, like batteries, operate by utilizing electrochemical reactions. Unlike a battery, in which chemical energy is stored within the cell, fuel cells generally are supplied with reactants from outside the cell. Barring failure of the electrodes, as long as the fuel, preferably hydrogen, and oxidant, typically air or oxygen, are supplied and the reaction products are removed, the cell continues to operate.
Fuel cells offer a number of important advantages over internal combustion engine or generator systems. These include relatively high efficiency, environmentally clean operation especially when utilizing hydrogen as a fuel, high reliability, few moving parts, and quiet operation. Fuel cells potentially are more efficient than other conventional power sources based upon the Carnot cycle.
The major components of a typical fuel cell are the hydrogen electrode for hydrogen oxidation and the oxygen electrode for oxygen reduction, both being positioned in a cell containing an electrolyte (such as an alkaline electrolytic solution). Typically, the reactants, such as hydrogen and oxygen, are respectively fed through a porous hydrogen electrode and oxygen electrode and brought into surface contact with the electrolytic solution. The particular materials utilized for the hydrogen electrode and oxygen electrode are important since they must act as efficient catalysts for the reactions taking place.
In an alkaline fuel cell, the reaction at the hydrogen electrode occurs between the hydrogen fuel and hydroxyl ions (OHxe2x88x92) present in the electrolyte, which react to form water and release electrons:
H2+2OHxe2x88x92xe2x86x922H2O+2exe2x88x92.
At the oxygen electrode, the oxygen, water, and electrons react in the presence of the oxygen electrode catalyst to reduce the oxygen and form hydroxyl ions (OHxe2x88x92):
O2+2H2O+4exe2x88x92xe2x86x924OHxe2x88x92.
The flow of electrons is utilized to provide electrical energy for a load externally connected to the hydrogen and oxygen electrodes.
In order that the oxygen reduction from air take place at a finite rate, it is necessary to incorporate some catalyst. In the Ovonic Regenerative fuel cells we have been using non-noble metal catalysts with great success. The so called oxygen reduction over-potential is significantly reduced by incorporation of such catalysts. Silver and cobalt based catalysts have been developed for this purpose. While silver catalysts work well the over-potential is about 50 mV higher than those based on the cobalt oxide based catalysts. Significantly, the use of cobalt oxide has enabled Ovonic fuel cells to utilize Stanford R. Ovshinsky""s concept of Valency Change. This involves the use of oxygen from the air only for chemically reoxidizing the electrochemically reduced product and thus get away from the kinetically difficult oxygen reduction reaction. By the choice of the specific redox reaction combined with appropriate matrix material it is possible to produce electrodes operating at higher potential ranges.
The present invention discloses an oxygen electrode having a porous metal matrix with the required porosity and built-in hydrophobic character. The unique character of the oxygen electrode is helpful in maintaining the air diffusion layer where electrode flooding needs to be avoided. On the other side of the oxygen electrode, where the active catalyst is going to be incorporated, the lesser amount of hydrophobicity allows controlled penetration of aqueous liquids. The varying layers of hydrophobicity within the electrode allow controlled deposition of the catalyst contained within an aqueous solution onto the electrode. This controlled catalyst deposition allows the catalyst to be deposited in the oxygen electrode at desired locations. This is important because it allows the catalyst to be deposited where it is needed most on the oxygen electrode.
The object of the instant invention is a double layer oxygen electrode impregnated with active catalyst material. The layered structure of the oxygen electrode promotes oxygen dissociation and absorption within the oxygen electrode. The layered structure of the oxygen electrode also allows impregnation of the active catalyst material into the oxygen electrode where the active catalyst material is needed most.
The present invention discloses an oxygen electrode including a porous first layer having a built-in hydrophobic character, a porous second layer having a greater built-in hydrophobic character than the porous first layer, and two current collector grids. The porous first layer and the porous second layer are positioned adjacent to each other. The current collector grids are placed outside of the two layers forming a sandwich structure.
The porous first layer of the oxygen electrode is composed of a carbon matrix. The carbon matrix is composed of polytetrafluoroethylene coated carbon particles containing approximately 15-25% polytetrafluoroethylene by weight. The porous first layer of the oxygen electrode may also contain 0-30% by weight of a peroxide decomposer. The peroxide decomposer may be MnO2, MnO, cobalt oxides, nickel oxides, iron oxides, and mixtures thereof. The porous first layer may also be impregnated with an active catalyst material. The active catalyst material may be deposited from AgNO3, a AgNO3/Ga(NO)3 mixture, a AgNO3/LiNO3 mixture, Co(NO3)2, a cobalt amine complex, NI(NO3)2, Mn(NO3)2, cyano complexes, organo-metallic complexes, amino complexes, citrate/tartrate/lactate/oxalate complexes, transitive metal complexes, and mixtures thereof. After impregnation, the active catalyst material forms submicron to nano particles within said carbon matrix. The porous first layer may be impregnated with the catalyst via chemical or electrochemical impregnation with or without vacuum, chemical vapor deposition, plasma vapor deposition or other impregnation techniques.
The porous second layer of the oxygen electrode is also composed of a carbon matrix. The carbon matrix contains polytetrafluoroethylene coated carbon particles containing approximately 40-60% polytetrafluoroethylene by weight.
The current collector grids each comprise at least one selected from the group consisting of mesh, grid, matte, expanded metal, foil, foam and plate (with or without perforations).
The oxygen electrode of the present invention may be prepared by first preparing a first carbon matrix having a built-in hydrophobic character, preparing a second carbon matrix having a built-in hydrophobic character greater than said first carbon matrix; and layering the first carbon matrix and the second carbon matrix between a two current collector grids. After placing the two layers between the current collector grids, a pressure of 2 tons per 100 cm2 is applied to the oxygen electrode followed by heat treating at 375 degrees Celsius for 15 to 30 minutes, followed by applying a pressure of 60 tons per 100 cm2 to the oxygen electrode. Resulting is a porous oxygen electrode having differing layers of hydrophobicity.
Once formed, the oxygen electrode is impregnated with an active catalyst material. The active catalyst material is deposited into the oxygen electrode layer having the least hydrophobicity. The active catalyst material may be impregnated from AgNO3, a AgNO3/Ga(NO)3 mixture, a AgNO3/LiNO3 mixture, Co(NO3)2, a cobalt amine complex, NI(NO3)2, Mn(NO3)2 cyano complexes, organo-metallic complexes, amino complexes, citrate/tartrate/lactate/oxalate complexes, transition metal complexes, and mixtures thereof. The active catalyst material forms submicron to nano particles within the oxygen electrode.
The oxygen electrode is preferably impregnated with the active catalyst material using a chemical impregnation process. The chemical impregnation process includes the steps of first preparing an aqueous solution of active catalyst material, dipping the oxygen electrode into the aqueous solution of active catalyst material, simultaneously pulling the aqueous solution of active catalyst material into the oxygen electrode via vacuum, drying the oxygen electrode at room temperature, heat treating the oxygen electrode at 50 degrees Celsius to remove any water from the oxygen electrode; and heat treating the oxygen electrode at 300-375 degrees Celsius to remove and decompose the nitrates remaining inside the oxygen electrode. Alternatively, the oxygen electrode may be impregnated with the active catalyst material using a chemical vapor deposition impregnation process or plasma vapor deposition impregnation process.